In many procedures, such as minimally-invasive surgery or catheter-based diagnosis and/or intervention, it is important for the physician to know the location of an instrument or probe, such as a diagnostic and/or therapeutic catheter, probe, arm, or other structure relative to the patient's internal anatomy. During cardiovascular catheterization procedures to address electrophysiologic problems, for example, a physician may steer an electrophysiology mapping catheter, typically under fluoroscopy, through a main vein or artery into the interior region of the heart that is to be treated. The physician then may determine the source of the cardiac rhythm disturbance (i.e., the targeted heart tissue) either strictly by anatomical considerations or by placing mapping elements carried by the catheter into contact with the heart tissue, and operating the mapping catheter to generate an electrophysiology map of the interior region of the heart. Having identified the targeted heart tissue, the physician then steers a radio frequency (RF) ablation catheter (which may or may not be the same catheter as the mapping catheter above) into the heart and places an ablation electrode in the blood stream against the targeted heart tissue carried by the catheter tip near the targeted heart tissue, and directs RF energy from the ablating element to ablate the tissue and form a lesion, thereby treating the cardiac disturbance. It is important that the contact between the electrode and the tissue be maximized to direct the RF energy toward the targeted heart tissue rather than through the blood stream.
It is known that the impedance between an electrode and tissue increases with an increase in contact between the electrode and the tissue. Based on this principle, prior art methods have taken impedance measurements from the electrode to ascertain when sufficient contact is established between the electrode and the targeted heart tissue for carrying out the ablation procedure. A baseline impedance measurement can be taken when the electrode is known to reside entirely within the blood stream, and contact with tissue is assumed to have occurred when the impedance has increased by a predetermined amount set empirically for a given system.
Besides ascertaining electrode-tissue contact for purposes of effecting sufficient tissue ablation or other diagnosis and/or intervention, it is sometimes desirable to determine the forces applied at the interfaces between electrodes and tissue structures, or the amount of electrode surface in contact with the tissue, to prevent or minimize the chance that the tissue will be inadvertently damaged or punctured by the interventional and/or diagnostic tools carrying the electrodes. While a physician can typically obtain some level of tactile feel for the force created between the instrument and tissue structures during manual manipulations of relatively light-weight instruments such as catheters within the patient, optimal resolution of the sensation maybe inadequate, and with larger instruments, manual sensation of distally-applied forces may be substantially impractical or impossible. Robotic systems that automatically manipulate catheters in response to movements of a control device at a remote user interface have recently been developed. Such systems are operated without direct manual manipulation of the instruments, and thus a physician cannot rely on directly-transmitted tactile feedback, but instead, may rely upon feedback provided by the robotic system, such as visual, audible, and/or tactile feedback, to maintain precision control over the subject instrument or instruments. It is preferred that such robotic systems be enabled with multiple means for determining the extent of contact or force between instrument electrodes and tissue.
Although the acquisition of impedance measurements has been generally successful in determining when an electrode has been placed in contact with tissue, the variation in impedance of tissue and blood between patients makes it difficult to accurately determine the extent of such electrode-tissue contact. Thus, during tissue ablation and other diagnostic and/or interventional procedures, firm effective contact between the electrode and tissue, as opposed to insufficient contact between the electrode and tissue, may not always be ascertained. With respect to preventing inadvertent damage to tissue, normal electrode-tissue contact, as opposed to contact that risks damage to tissue, may not always be ascertained.
There thus remains a need for an improved system and method for ascertaining contact between an electrode and tissue for various configurations of diagnostic and/or interventional instruments in various clinical settings.